Unlike SP2, there's no installation tool for installing SP3 on an SBS 2008. Fortunately, the installation is pretty straight-forward. The steps to perform depend largely on whether or not your SBS 2008 server is running Forefront Security or not. If it's not, simply install SP3 like you normally would. The installation will succeed without additional tasks. If you are running Forefront Security, you will have to disable it before installing SP3, then re-enable it when installation has completed.